In a mobile phone system mobiles, which communicate with a base station are often located at different distances from the base station. That means the time it takes for information sent by the mobile station (MS) to arrive at the base station is different. Each mobile station is allocated a timeslot, i.e. it is only allowed to transmit during a specific period of time. This is necessary since if two mobile stations were to transmit at the same time it would not be possible for the base station to decode the transmissions. Therefore it is important that the transmissions from the mobile stations arrive at the base station, aligned with the timeslot structure there, i.e. that they do not overlap due to different time delays.
The issue is solved by letting the mobile stations transmit special information sequences, which are used by the base station to calculate an individual time delay. The individual time delay calculated is used to order the mobile stations to offset their transmissions in time such that their transmissions arrive at the base station aligned to the timeslot structure. The message sent to the mobile stations is called a timing advance message (TA-message).
FIG. 1 shows the standard mapping of the Timing Advance (TA) messages onto a multi-frame structure. An Enhanced General Packet Radio Service multi-frame, EGPRS, or General Packet Radio Service GPRS multiframe, consists of 52 Time Division Multiple Access (TDMA) frames, which are divided into 12 Radio Link Control/Medium Access Control (RLC/MAC) blocks plus 4 single bursts. When either GRPS or EGPRS is referred to, hereafter (E)GPRS is used for short.
In an (E)GPRS network several mobile stations can be multiplexed on the same packet data channel (PDCH). The maximum number of mobile stations that can share a PDCH is 16. Every TA-message is interleaved and divided into 4 bursts, which are spread over 2 multi-frames. Each one of the TA-messages contain timing advance information for all mobile stations, but the timing advance information is only updated for 4 mobile stations in each TA-message. Hence 4 TA-messages must be sent in order to update all mobile stations. Thus a mobile station is only required to listen to every 4:th TA-message, however if the mobile station isn't able to decode the message it tries to retrieve the information from the next. The transmission of all TA-messages takes approximately 2 seconds.
An adaptive antenna system is defined to be a system that is able to change its characteristics to changes in the network. Such a system has several interesting properties. One of the most important features on an adaptive antenna system is that the base station is able to detect the direction to the mobile station and can thus transmit dedicated information in an antenna beam towards a desired mobile station. An antenna beam is defined as any signal transmission covering only a part of a cell, and a cell is defined as the total coverage area of a base station. An international application WO 01/31810 A1 is also related to the basic problem of simultaneously transmitting information to mobile stations located in different beams. However the number of mobile stations concerned and the coding of the information sent is different in that application compared to the present one.
All mobile stations assigned to the same Packet Data Channel (PDCH) will be assigned a unique Timing Advance Index (TAI) during the Packet Uplink or Packet Downlink Assignment. The TAI tells the mobile when to transmit its access burst, which is used by the network to calculate the timing advance. The update of the timing advance is sent to the mobile station in the next TA message sent after the burst is received on the uplink. For instance, a mobile that transmits its burst on T1 (see FIG. 1) will receive the updated value in TA message 2.
The timing advance procedure is thoroughly described in the document having the title “Overall description of the GPRS radio interface” [1].